Radio-labeling of pembrolizumab with the positron emission tomography (PET) radionuclide 89Zirkonium (89Zr) enables serial non-invasive imaging and quantification of distribution of PD-1 in melanoma patients. By performing a 89Zr-pembrolizumab-PET scan prior to treatment with pembrolizumab, the uptake of the tracer in the tumor lesions and normal organ distribution can be evaluated, this could lead to new insights about heterogeneity of PD-1 expression, as well as the use of a 89Zr-pembrolizumab-PET as a complementary tool for patient selection in the future.